1. Field of the Invention
The present invention relates to an intermediate transfer member which is used for an electro-photographic copying machine, a printer, a facsimile machine, a combination of such machines or the like that is provided with a tandem type color image forming device having a photoreceptor for each of a plurality of colors, which photoreceptors are each provided with a developer unit and disposed at the intermediate transfer member, and to a method of fabricating the intermediate transfer member.
2. Description of the Related Art
In an image forming device in which an electro-photographic system is applied, a latent image holding member which consists of an inorganic or organic photoconductive photosensitive material (which can be referred to simply as a xe2x80x9cphotoreceptorxe2x80x9d hereinafter), is charged uniformly. Subsequently, an electrostatic latent image is formed on the latent image holding member by light of a laser or light of a light emitting diode (LED) which light is obtained by modulation of image signals. The electrostatic latent image is developed with the charged toner to thereby form a visualized toner image. The toner image is electrostatically transferred, via an intermediate transfer member or directly, onto a transfer material such as paper so that a required image is obtained. Specifically, there has been known an intermediate transfer process comprising a first transfer process and a second transfer process. In the first transfer process, the toner image formed on the latent image holding member is first transferred onto an intermediate transfer member. In the second transfer process, the toner image transferred onto the intermediate transfer member is then transferred onto paper.
Examples of raw materials used for forming an endless belt which is used in the image forming device employing the aforementioned intermediate transfer process include: a polycarbonate resin (PC), polyvinylidene fluoride (PVDF), polyalkylenephthalate (PAT), a blended material thereof and a thermoplastic resin such as an ethylenetetrafluoroethylene copolymer (ETFE), which raw materials can form a semiconductive endless belt.
Japanese Patent No. 2560727, Japanese Patent Application Laid-Open (JP-A) No. 5-77252, and the like disclose an intermediate transfer member in which ordinary carbon black is dispersed as electrically conductive powder in a polyimide resin.
Tandem type intermediate transfer process color image forming devices and the like have been investigated as novel image forming devices employing this kind of intermediate transfer member (see FIG. 1). For example, in the device shown in FIG. 1, a plurality of photoreceptors 9 each having a developer unit 15 for each color is disposed at the intermediate transfer member 16. Images which are separated for each color and respectively formed on each of the photoreceptors 9 are transferred onto the intermediate transfer member 16 so that a full-color image is formed on the intermediate transfer member 16 with one cycle. The full color image is then transferred onto a member all at once onto a transfer-receiving member such as paper or the like.
Conventionally, as another method for transferring an image to a member to which transfer is to be made (e.g., paper), there has also been known a transfer drum method, in which a member to which transfer is to be made (e.g., paper) is wound onto a transfer drum and the image on the photoreceptors is transferred, for each color to the paper to which transfer is to be made. In this transfer drum method, as opposed to the tandem type intermediate transfer method, when the image is transferred from the photoreceptors to the paper that is wrapped around the transfer drum, it has been necessary to rotate the transfer drum several times (e.g. four times). However, with the tandem type intermediate transfer method, an image for all colors can be transferred from a plurality of photoreceptors with a single rotation of the intermediate transfer body, whereby it is possible to improve transfer speed. Furthermore, the tandem type intermediate transfer method has the advantage of not needing to specify the body to which the final transfer is made. Accordingly, the tandem type transfer method is being examined as a promising transfer method.
However, the intermediate transfer member used in this tandem type intermediate transfer process has a diameter that is larger than that of a conventional intermediate transfer belt, and is provided with plural, for example, from two to four, independent color developer units and photoreceptors. For this reason, when the tandem type intermediate transfer process is used, it is necessary to design an image forming device capable of preventing color unevenness such as color drift and color misregistration, for each color. Namely, the intermediate transfer member that is provided at such an image forming device needs to have extremely high accuracy and a high level in planarity. For example, when the planarity is low, adhesiveness between a drive roll 11 for driving a belt and the intermediate transfer member varies by portions, and the moving speed of the belt thereby changes irregularly, thus differentiating the moving speed of the intermediate transfer member with respect to each of the photoreceptors which are provided with a developer unit for each color. As a result, there arises a serious problem in that image matching with a high level of accuracy becomes impossible, thereby causing image faults such as color unevenness. With the miniaturization of image forming devices, internal components or members of recent image forming devices tend to be closer to one another. When the planarity of the intermediate transfer member is low and irregularities or float portions are formed, various difficulties occur. The float portions approach other members and electrical discharge occurs, preventing portions of the image from forming. Further, the end portions of the intermediate transfer member are damaged by contact with other members in the vicinity thereof. In the worst case situation, the intermediate transfer member is caught by other members and broken.
For the conventional intermediate transfer member which is used for the aforementioned tandem type color image forming device and which is formed from a thermoplastic resin of the conventional technology, resistance value, thickness, surface roughness, planarity, and the like were uneven and not accurate enough. In Japanese Patent No. 2560727 and JP-A No. 5-77252 is disclosed a tandem type color image forming device in which in-plane variation of resistance value of the intermediate transfer member is improved to some degree. However, surface roughness and planarity required for the tandem type color image forming device were not satisfactory.
To solve the above-mentioned problems in the conventional technology, the following objects should be achieved.
An object of the present invention is to provide an intermediate transfer member which is used for a tandem type intermediate transfer image forming device and which has high planarity and is thereby able to form a high quality transfer image, and a fabrication method thereof. Another object of the present invention is to provide an intermediate transfer member in which electrical resistance thereof is uniform and planarity and thickness are accurate, and a fabrication method thereof.
First aspect of the present invention is an intermediate transfer member for use in a tandem type color image forming device. The intermediate transfer member comprises an endless belt shape and including as a main component a polyimide resin. The intermediate transfer member has a planarity of at most 5 mm, when the intermediate transfer member is wrapped around two substantially parallel shafts and a tension load of 4 kg is applied to one of the shafts to longitudinally stretch the intermediate transfer member,
Second aspect of the present invention is a fabricating method for an intermediate transfer. The method comprises the steps of preparing a solution of polyamide acid, supplying the solution to an inner surface of a cylindrical mold and forming a film of the solution thereat, and hardening the film by heating so as to form therefrom a belt capable of retaining a belt shape when unsupported. The method further comprises the steps of removing the belt from the cylindrical mold, disposing the belt at an outer surface of a metal cylinder, heating the belt to an elevated temperature and carrying out an imide conversion reaction by maintaining the belt at a temperature of from 220 to 280xc2x0 C. which is equal to or less than the temperature for at least 0.5 hours.
Third aspect of the present invention is a fabricating method for an intermediate transfer. The method comprises the steps of preparing a solution of polyamide acid, supplying the solution to an inner surface of a cylindrical mold and forming a film of the solution thereat, and hardening the film by heating so as to form therefrom a belt capable of retaining a belt shape when unsupported. The method further comprises the steps of removing the belt from the cylindrical mold, disposing the belt at an outer surface of a metal cylinder having surface roughness of from 1.2 to 2.0 xcexcm, and heating the belt for carrying out an imide conversion reaction.